Recirculation cyclones for dedusting and dry gas cleaning

ABSTRACT

Recirculation cyclones for dedusting and dry gas cleaning—comprising a reverse-flow cyclone collector (a, b, H 1 , D 1 , h, D b , D e1 , s 1 ) and a straight-through cyclone concentrator (D e1 , H 2 , D 2 ; D e2 , s 2 , D v ), located in series and with recirculation—characterised by the collector being located upstream from the concentrator and by a recirculation line (D v ), that recirculates a fraction of the flue gases from the concentrator to the collector, by means of a fan, ejector or venturi. Dedusting and dry gas cleaning methods, characterised by making the flue gases pass through such a device. Utilisation of those methods and device to dedusting exhaust gases from diesel combustion or to dedusting and dry gas cleaning acid gases like HCl, HF, SO 2  and/or NO x  by an injection of a solid sorbent. The efficiency is always larger than that of a reverse flow cyclone alone, with the same geometry and size as the collector, and than that of recirculation systems with the concentrator upstream the collector, with comparable geometries and sizes.

TECHNICAL DOMAIN

[0001] The present invention, shown schematically in FIG. 1, is a recirculation system employing cyclones, and belongs to the class of equipments used for dedusting and dry-gas cleaning.

[0002] As a matter of fact, cyclones are dedusters used in many types of industries with two purposes: removal of particulate matter emitted from processes, before release to the atmosphere (pollution control and/or raw materials recovery), or as reactors for the removal of acid components from flue gases by dry injection of appropriate sorbents. These reactors are frequently followed by bag filters for fine particle recovery.

Current State of the Art

[0003] Industrial cyclones vary in size and shape, where the most common are of the reverse-flow type.

[0004] The first reverse-flow cyclones date from the 19th century, and their design has evolved mostly from empirical observation.

[0005] Theoretically, cyclone efficiency increases with gas flow rate, but in practice there is a limit beyond which efficiency decreases. This is due to saltation or reentrainment (Licht, 1980), much like what happens in sand dunes which are blown by a strong wind.

[0006] To remedy this problem, partial gas recirculation has been proposed, using a fan or appropriate ejector (FIG. 2. Berezowski and Warmuzinski, 1993). Similar examples may be observed under U.S. Pat. No. 3,254,478.

[0007] To increase cyclone efficiency, these may be connected in series, as long as correctly designed, but with the cost of increased pressure drop and operating costs (Salcedo, 1993).

[0008] Thus, cyclone recirculation systems were developed, composed by a straight-through cyclone (from now on referred as the concentrator) upstream from a reverse-flow cyclone (from now on referred as the collector), with partial recirculation to the concentrator, using some fan. These are schematically shown in FIG. 3 (Crawford, 1976; Svarovsky, 1981; Wysk et al., 1993). The system proposed by these last authors has been granted U.S. Pat. No. 5,180,486. The gas to be treated enters the concentrator through a tangential entry, rises in a vortex flow and is divided in two parts: one that escapes to the atmosphere and the other that enters the collector, also through a tangential entry. Here the gas follows a descending vortex, until it changes direction due to the established pressure field (thus the name of reverse-flow) exiting on top by a cylindrical tube, the vortex finder, of some appropriate length. As they follow the downward vortex, solid particles are thrown to the wall due to centrifugal forces, and end up falling on the cyclone bottom, being separated from the gas. The gas and remaining particles exiting the collector are recycled to the concentrator through a centrifugal fan.

[0009] These systems may be much more efficient than single reverse-flow cyclones (collectors), and their collection efficiency is given by: $\begin{matrix} {\eta = \frac{\eta_{con}\eta_{col}}{1 - \eta_{con} + {\eta_{con}\eta_{col}}}} & (1) \end{matrix}$

[0010] where η_(con) and η_(col) are respectively the concentrator and collector efficiencies. This equation shows that for η_(con)≦η_(col), the system efficiency is always lower than that for a singe collector (η_(col)), but that for η_(con)>η_(col), the system efficiency is always larger. Thus, these systems are only interesting whenever the concentrator efficiency is significantly higher than the collector efficiency. This concept is schematically shown in FIG. 4.

[0011] Summing up, there are in the marketplace cyclone recirculation systems that may be, under some circumstances, significantly more efficient than single reverse-flow cyclones, which use a concentrator upstream from the collector, with recirculation from the collector to the concentrator through an appropriate fan or ejector. However, as shown, they are not always more efficient than single collectors.

[0012] There are also gas cleaning devices that employ dry sorbent injection of finely divided powders, but they still have high investment costs (Carminati et al., 1986; Heap, 1996; Fonseca et al., 1998).

Objectives of the Invention

[0013] The present invention has as main objective to increase the collection efficiency of cyclone dedusters with recirculation, even when the concentrator efficiency drops below the collector efficiency.

[0014] It is also an objective of the present invention to make available a highly efficient system for the dedusting and acid gas cleaning of flue gases.

[0015] Additional objectives will become obvious following the remaining description and from the proposed claims.

Describing the Invention

[0016] The proposed objectives are achieved by considering a system of recirculation cyclones, where the collector is located upstream of the concentrator, and the recycling is made by an appropriate fan, venturi or ejector.

[0017] With the objective of obtaining cyclone systems which are more efficient than those available in the marketplace, but with similar investment and operating costs, which may be used at high temperatures and pressures or for dry gas cleaning, a study has been initially made on the most efficient configuration.

[0018] It is verified that, although the system components are essentially those from the previous state of the art, inverting their relative position makes the proposed system always more efficient than single reverse-flow cyclones or than recirculation systems with the concentrator located upstream from the collector. As the concentrator and collector are in series, the investment and operating costs are similar than those from recirculation systems with the collector downstream from the concentrator. Employing a venturi for recirculation makes it possible to use this system at very high temperatures (>1000° C.). For larger flow rates, appropriate fans or ejectors may be used. These systems may also be used for acid dry gas cleaning, since reverse-flow cyclones may be excellent reactors for this purpose.

A New Approach

[0019] By simple theoretical arguments, the solution to this problem is a system where the collector is located upstream from the concentrator. The global efficiency for this system is given by: $\begin{matrix} {\eta = \frac{\eta_{col}}{1 - \eta_{con} + {\eta_{con}\eta_{col}}}} & (2) \end{matrix}$

[0020] As the denominators of Eqs. 1 and 2 are the same, and as the numerator from Eq. 2 in always larger than that from Eq. 1, the efficiency of the proposed system is always higher than that from recirculation systems available in the marketplace. This concept is shown in FIG. 5.

DESCRIBING THE FIGURES

[0021]FIG. 1 is a schematic representation of the proposed system, made-up by a reverse-flow cyclone (collector), followed by a straight-through cyclone (concentrator), showing its main dimensions, and by some recirculation means that may be a fan, an ejector or a venturi.

[0022]FIG. 2 is a schematic representation of a reverse-flow cyclone with recycling through a fan. This system has been used, as per the previous state of the art, to minimize particle reentrainment due to excessive velocities.

[0023]FIG. 3 is a schematic representation of a recirculation system, made-up by a straight-through cyclone (concentrator), followed by a reverse-flow cyclone (collector), with the recirculation made by a fan, as per the previous state of the art.

[0024]FIG. 4 shows the global efficiency for the system depicted in FIG. 3. The system efficiency is only better than that of a single collector when the concentrator efficiency is larger than the collector efficiency.

[0025]FIG. 5 shows the global efficiency for the system depicted in FIG. 1. The system efficiency is always better than that of a single collector.

[0026]FIG. 6 compares the grade-efficiencies of a single collector with that of the proposed system, for laboratory-scale collectors and concentrators (0.02 m), gas flow rate of 3.3×10⁻⁴ m³s⁻¹ and unit density spherical particles.

[0027]FIG. 7 shows that a venturi is capable of providing for significant recirculation, if this is the recirculation means employed.

ADVANTAGES

[0028] As previously stated, besides the proposed system efficiency being always larger than that from the current state of the art (FIG. 3), where the concentrator is located upstream from the collector, for comparable geometries and sizes—as it was previously seen by comparing Eqs. 1 and 2 and also by comparing FIGS. 4 and 5—the proposed system has an efficiency always larger than that from a single collector, unlike what happens whenever the concentrator is located upstream from the collector, as referred above.

[0029] The proposed system may also be used in advantage over existing reactors for dry gas cleaning (spray dryers or venturi scrubbers) for acid gas cleaning (HCl, HF, SO₂ and NO_(x)), where very compact and high efficiency units may be designed.

Describing the Invention in Detail

[0030] The hereby proposed recirculation systems, that comprise two cyclones, one of the reverse-flow type (collector) and the other a straight-through cyclone (concentrator), is characterized by the collector being placed upstream from the concentrator, with partial recirculation from the concentrator to the collector made with a fan, a venturi or ejector. The collector has a rectangular tangential entry of sizes a and b, the first being parallel to the cyclone axis, or a circular section of equivalent area; a body of height H₁, with an upper cylindrical portion of diameter D₁ and height h, with a lower inverted cone with smaller base diameter D_(b); and a cylindrical vortex finder, of diameter D_(e1) and height s₁. The cyclone concentrator has a tangential entry of essentially circular section, of diameter D_(e1); a cylindrical body of height H₂ and diameter D₂; a cylindrical vortex finder of diameter D_(e2) and height s₂; and two exits, one being tangential and essentially circular, with diameter D_(v), and the other axial with diameter D_(e2). The venturi, if this is the recirculation means employed, is any standard venturi type with adequate dimensions, calculated by conventional methods.

[0031] The three components are connected as follows: the gas to be cleaned enters the reverse flow cyclone, which captures some particles; the escaping particles follow with the total gas to the straight-through cyclone (concentrator), and part of the gas concentrated with uncaptured particles is recycled to the reverse flow cyclone by means of an auxiliary fan, venturi or ejector.

[0032] To better understand these phenomena, the proposed system was modelled using the Mothes and Loffler (1998) theory, which is presently the best model available to predict cyclone performance (Clift et al., 1991; Salcedo, 1993; Salcedo and Fonseca, 1996; Hoffmann et al., 1996; Salcedo and Coelho, 2000). FIG. 6 shows the predicted grade efficiency curves (efficiency for each particle size) for the proposed system, as compared with the single collector, for a laboratory-scale system, both treating the same particles and for the same gas flow rate, where decreases in emissions above 50% are expected.

[0033] The proposed system has the following characteristics that differentiate it from competing systems available in the marketplace:

[0034] Efficiency always larger than that of a reverse flow cyclone with the same geometry and size as the collector;

[0035] Efficiency always larger than that of recirculation systems with the concentrator upstream the collector, as long as geometries and sizes are comparable;

[0036] Recirculation through a fan, venturi or ejector.

[0037] May be used either as dedusters or as acid dry gas cleaning systems;

[0038] May be used at high temperatures, provided a venturi or ejector is employed for recirculation purposes;

[0039] Absence of moving parts as long as a venturi or ejector is employed for recirculation purposes.

[0040] Thus, the present patent submission proposal refers to a system of two cyclones, used for dedusting or dry gas cleaning, where the collector is a reverse-flow cyclone upstream from the straight-through cyclone concentrator, with partial recirculation by venturi, fan or ejector, as well as to the respective method of dedusting or dry gas cleaning.

PRACTICAL EXAMPLES

[0041] A laboratory-scale prototype was built to demonstrate the recirculation capabilities of a venturi, and this has been clearly shown (FIG. 7).

[0042] Thus, it is predicted that the proposed system may reduce significantly emissions when compared with single reverse-flow cyclones or with recirculation systems with the concentrator located upstream of the collector. This has already been shown at a laboratory-scale, where a reverse-flow cyclone with 0.02 m inside diameter and geometry according to patent PT102166 (which is referred in FIG. 6), has a collection efficiency of 80% for Ca(OH)₂ (lime) with 1.37 μm of mean mass diameter, at a gas flow rate of 20 lmin⁻¹.

[0043] Connecting to the collector a straight-through concentrator with 0.02 m inside diameter and making partial recirculation to the collector with a venturi of 0.002 m throat diameter, as per FIG. 1, the collection efficiency increases to 96%. Reductions in emissions of 80% (from 20 to 4%) are then possible. Thus, by using very high efficiency geometries for the collector (for example, that described in PT102166) allows the proposed system to compete with much more expensive dedusters (spray and absorption towers, venturis, pulse jet bag filters), except in what refers to extremely small particles (<0.5 μm), with the added advantage that they may be used at very high temperatures and for acid gas cleaning by dry injection of a solid sorbent. The development of dedusting systems with collection efficiencies well above those from single reverse-flow cyclones, using simple and economical technologies, especially for particle sizes below 2-3 μm, has a great potential for industrial application. Several industries (wood, metals, cements, chemicals), and fuel boilers could benefit from economical and efficient dedusters to avoid the need of using much more expensive devices, such as pulse jet bag filters.

[0044] Likewise, the automotive industry, as it refers to emissions control of particulates from diesel vehicles, could benefit from a simple equipment such as the proposed one, which may be used at high temperatures and does not have moving parts.

[0045] The proposed system has also clear advantages over reactors usually employed for acid gas cleaning (HCl, HF and SO₂), where extremely compact and efficient units may be designed, both in the removal of acid gases and in the rate of use of solids injected as a dry powder, due to the partial recirculation of the unreacted sorbent.

References

[0046] Berezowski, M. and K. Warmuzinski, ‘Recycling as a means of controlling the operation of cyclones’, Chemical Engineering and Processing, vol,32, 345-347, 1993.

[0047] Carminati. A., A. Lancia, D. Pellegrini and G. Volpiccelli, ‘Spray dryer absorption of HCl from flue gas’, Proc. 7^(th) World Clean Air Congr., 426, 1986.

[0048] Clift, R., M. Ghadiri and A. C. Hoffman, ‘A Critique of Two Models for Cyclone Performance’, AlChE J., vol.37, 285-289, 1991.

[0049] Crawford, M., ‘Air Pollution Control Theory’, McGraw-Hill, 1976.

[0050] Fonseca, A. M., José J. Órfão and Romualdo L. Salcedo, ‘Kinetic modeling of the reaction of HCl and solid lime at low temperatures’, Ind. Eng. Chem. Res., vol.37, 4570-4576, 1998.

[0051] Heap, B. M., ‘The continuing evolution and development of the dry scrubbing process for the treatment of incinerator flue gases’, Filtr. Sep., vol. 33, 375, 1996.

[0052] Hoffmann, A. C., M. de Groot and A. Hospers, ‘The effect of the dust collection system on the flowpattern and separation efficiency of a gas cyclone’, Can. J. Chem. Eng., vol.74, 464-470, 1996.

[0053] Licht, W., ‘Air Pollution Control Engineering-basic calculations for particulate collection’, Marcel Dekker, New York and Basel, 1980.

[0054] Mothes, H. and F. Loffler, ‘Prediction of particle removal in cyclone separators’, International Chemical Engineering, vol. 28, 231-240, 1988.

[0055] Salcedo, R. L., ‘Collection Efficiencies and Particle Size Distributions from Sampling Cyclones - Comparison of Recent Theories with Experimental Data’, Can. J. Chem. Eng., vol.71, 20-27, 1993.

[0056] Salcedo, R. L. and A. M. Fonseca, ‘Grade-efficiencies and particle size distributions from sampling cyclones’, Mixed-Flow Hydrodynamics, Cap. 23, 539-561, P. Cheremisinoff (ed.), Gulf Publishers, 1996.

[0057] Salcedo, R. L. and M. A. Coelho, ‘Turbulent Dispersion Coefficients in Cyclone Flow: an empirical approach’, Can. J. Chem. Eng., August 2000.

[0058] Svarovsky, L., ‘Solid-Gas separation’, Elsevier Scientific Publishing Co.. NY, 1981.

[0059] Wysk, S. R., L. A. Smolensky and A. Murison, ‘Novel particulate control device for industrial gas cleaning’, Filtration & Separation, January/February. 29-31, 1993. 

1- Recirculation cyclones for dedusting and dry gas cleaning—comprising a reverse-flow cyclone collector (a, b, H₁, D₁, h, D_(b), D_(e1), s₁) and a straight-through cyclone concentrator (D_(e1), H₂, D₂; D_(e2), s₂, D_(v)), located in series and with recirculation—characterised by the collector being located upstream from the concentrator and by a recirculation line (D_(v)), that recirculates a fraction of the flue gases from the concentrator to the collector. 2- Cyclones according to claim 1, with the recirculation made by a fan. 3- Cyclones according to claim 1, with the recirculation made by an ejector. 4- Cyclones according to claim 1, with the recirculation made by a venturi. 5- Dedusting method characterised by making the flue gases pass through a device as per the claim
 1. 6- Dedusting and dry gas cleaning method, according to claim 5, characterised by an injection upstream from the collector, venturi, fan or ejector of an appropriate solid sorbent. 7- Utilisation of the device as per the claim 1 and of the method as per claim 6, for flue gas dedusting and dry gas cleaning. 8- Utilisation according to claim 7, characterised by the gases being acid gases, namely HCl, HF, SO₂ and/or NO_(x). 9- Utilisation of the device as per the claim 1 and of the method as per claim 5, for the dedusting of exhaust gases from diesel combustion. 